1. Field of the Invention
The present invention relates to a mobile wireless terminal such as a portable telephone unit or the like, in particular, to a mobile wireless terminal such as a portable telephone unit having an adaptive equalizer such as a waveform equalizer. The present invention also relates to a power controlling method for such a mobile wireless terminal.
2. Description of the Related Art
Since a mobile wireless terminal is driven with a battery, one of essential performances thereof is low power consumption. In particular, while the user travels with a mobile wireless terminal, the power thereof is usually turned on. Thus, when the mobile wireless terminal is in standby state or in out-of-service-area state, it is important to reduce the power consumption to prolong the operation time of the terminal. In addition, as the transmission rate becomes high, an adaptive equalizer that consumes large power is used in the mobile wireless terminal along with the conventional demodulator. In the mobile radio terminal using such a structure, the power control of the adaptive equalizer is important.
FIG. 1 is a block diagram showing the outlined structure of a mobile wireless terminal that performs the power control of the adaptive equalizer. The mobile wireless terminal detects a transmission radio wave (carrier) received from a base station. Only when the mobile wireless terminal has received the transmission radio wave, it causes the adaptive equalizer to operate. The mobile wireless terminal comprises an RF receiving portion 1, a channel switching controlling portion 2, a carrier detecting portion 3, a waveform equalizing portion 5, and a unique word (UW) detecting portion 6 as principal portions.
A radio wave transmitted from a base station is received by an antenna 1a. An RF signal received from the antenna 1a is supplied to an RF receiving portion 1. The RF receiving portion 1 receives the RF signal on a designated channel (at a designated reception frequency), converts the reception signal into a digital signal, and supplies the digital signal to the waveform equalizing portion 5. In addition, the RF receiving portion 1 supplies a signal that represents the level of the electric field of the received transmission radio wave to the carrier detecting portion 3. The field level signal is obtained by converting an output signal of a log amplifier of the RF receiving portion 1 into a digital signal.
The carrier detecting portion 3 compares the input field level signal with a predetermined threshold value. When the field level exceeds the threshold value, the carrier detecting portion 3 supplies a carrier detection signal to the waveform equalizing portion 5. When the carrier detecting portion 3 has not received a carrier for a predetermined time period, the carrier detecting portion 3 supplies a channel switch request signal to the channel switching controlling portion 2 so as to switch the reception channel of the RF receiving portion 1 into another reception channel.
When the channel switching controlling portion 2 receives the channel switch request signal from the carrier detecting portion 3, the channel switching controlling portion 2 switches the current channel (reception frequency) of the RF receiving portion 1 to another reception channel. This switching operation is performed by varying the frequency of a synthesizer of the RF receiving portion 1 (the synthesizer converts a frequency between a transmission/reception frequency and an intermediate frequency).
The waveform equalizing portion 5 equalizes a waveform distortion of the digital signal received from the RF receiving portion 1. Only when the carrier detection signal is received from the carrier detecting portion 3, the power of the waveform equalizing portion 5 is turned on. An output signal of the waveform equalizing portion 5 is input to a demodulator (not shown) and the UW detecting portion 6.
The UW detecting portion 6 detects a UW from a reception signal sequence that is output from the RF receiving portion 1 through the waveform equalizing portion 5. Corresponding to the UW detected by the UW detecting portion 6, a reference timing of a reception frame (for example, TDMA frame) is extracted. Corresponding to the frame reference timing, the synchronization of the reception frame is acquired.
In the above-described mobile wireless terminal, only when a carrier is detected, the power of the adaptive waveform equalizing portion 5 is turned on. Thus, in the standby state and in the out-of-service-area, the power consumption of the mobile wireless terminal is reduced.
As another related art reference disclosed in for example Japanese Patent Laid-Open Publication No. 2-82731, a waveform equalizing portion is composed of a first block and a second block. The power of the first block is always turned on. In contrast, the power of the second block is turned on only when a carrier is detected.
However, the mobile wireless terminal shown in FIG. 1 has the following problems.
When the radio reception state is bad and thereby the carrier/noise ratio (C/N ratio: ratio of power of carrier and noise) is bad, a carrier is frequently received without a received burst. When a mobile wireless terminal 11 of TDMA/TDD system is in out-of-service-area state as shown in FIG. 2, the mobile wireless terminal 11 cannot receive a down burst. In this case, the mobile wireless terminal 11 frequently detects a carrier corresponding to up bursts of other mobile wireless terminals 12 and 13. Thus, the mobile wireless terminal 11 frequently detects a carrier corresponding to signals received from other than the base station, the waveform equalizing portion unnecessarily operates and thereby remarkably consumes the power of the battery.
In addition, the mobile wireless terminal cannot determine whether a received carrier is a signal received from a relevant base station, and whether a signal is received from another terminal, or noise. Thus, the mobile wireless terminal cannot determine a channel switch timing when should be switched to another carrier channel. Consequently, the waiting time on the same channel unnecessarily becomes long.
In Japanese Patent Laid-Open Publication No. 1-252033, another method is disclosed. In the method, a control signal is acquired in the condition that the current reception channel is not varied for a predetermined time period. After it is determined whether a control signal has been received in out-of-service-area state or has temporarily deteriorated, the control signal or another control signal is acquired corresponding to the determined result. However, a practical means for applying such a method to the power control of the waveform equalizer has not been accomplished.